1. Field of the Invention
This invention relates to a limited movement roller system for safely and predictably moving large loads.
2. Description of Related Art
Ancient Egyptians allegedly moved large stone blocks by placing cylindrical rollers beneath them and then manually urging the blocks along. This rolling procedure required that the rollers emerging from the rear of the stone be manually lifted and replaced in front. This roller replacement protocol has been automated in commercially available roller units that allow continuous movement of heavy machinery under the action of pry bars, come-a-longs, winches, or manual push efforts. Unfortunately, when slopes or asperities are encountered, these heavy loads may accelerate uncontrollably or steer themselves in unsafe directions when the roller units become reoriented. It is therefore desirable to cause the locomotion of the machinery in inchmeal fashion by intermittently braking the system while the roller units are manually reset.
A typical roller unit is illustrated in FIGS. 1 and 2 where cylindrical rollers are mounted along a roller chain that circulates around a load bearing platform that supports a superstructure which in turn upholds a heavy machine. These roller units are often referred to as roller skids or skates. The rollers transfer compression loads between the ground surface and the bottom of the load bearing platform. Off-the-shelf units are available in capacities from 4 tons to 100 tons. Roller units are typically symmetrical longitudinally and transversely.
Existing roller systems typically use four roller units which are inserted beneath a load, such as a boiler, by jacking up the corners or other hard points. The roller units are seldom attached to the boiler which may be propelled by horizontal forces generated by shoving, winching, or prying. The boiler is steered by rotating the individual roller units about an imaginary vertical axis. This is generally accomplished manually using a three to five foot long lever with a T-bar handle. The steering lever is temporarily affixed to either end of the roller unit. A swivel bearing is sometimes added to the top of the roller unit to minimize rotational resistance and improve its steering capability.
Moving large masses on horizontal homogenous surfaces that are free of asperities may be safely accomplished by any horizontal force system that may be instantaneously interrupted, e.g., manual pushing or pry bars. As ideal conditions degenerate the following may be experienced: sloped surfaces, ramps, and inclines; textured and anisotropic surfaces (broom finished concrete); nonhomogeneous floors composed of multiple materials (brick, wood, steel, etc.); weak floor spots, expansion joints and drains; uneven surfaces; and/or dirty and debris laden floors.
When significant slopes are encountered, heavy loads may accelerate uncontrollably and this action may be exacerbated by spring-like propelling devices such as come-a-longs or winches. Roller manufacturers generally recommend that holdback devices be used on inclined surfaces. Surface imperfections generally contribute to the propensity of roller units to realign themselves and steer the moving machinery in unsafe directions. To help maintain control of the loads, roller manufacturers generally advocate the following precautions: constant monitoring of the rollers; moving slowly at all times; and absolute cleanliness of moving surfaces.
It is not unusual for movers to rig machinery with various winch-like devices to control their movement. Under general conditions, such as an elephant on an icy slope as shown in FIG. 3, holdback rigging is not elementary. There are fundamental difficulties with existing holdback technology. For example, when simultaneously pulling and holding back with two come-a-longs, the cables must be collinear; otherwise, a moment is introduced that will tend to rotate the elephant. If three nonparallel cables are used to restrain the beast, their lines of action must all intersect at a point; otherwise once again, they will rotate the elephant. The four cables shown in FIG. 3 completely fix the position of the elephant against rotation and translation. On the other hand, loosening and tightening the cables to cause the creature to move along a desired path in a specific orientation is a daunting exercise. It should be noted that some four-line rigging systems will not fully restrain a load. In addition, the location and structural integrity of available tie off points cannot be taken for granted.